An optical fiber and a bundle of optical fibers are variously modified in accordance with the conditions under which they are used. However, a tension member must be used with an optical fiber to assure a high strength in some cases. When water permeates an optical fiber cable, its strength may be degraded. When an optical fiber cable is to be installed in the bottom of a sea or the bottom of the water, in order to assure a sufficient installation tension and a high water resistance, an optical fiber cable must be used in a jacket structure in which an optical fiber cable is covered with a thin metal tube.
In this manner, there is provided an apparatus and method of continuously manufacturing a metal tube covered optical fiber cable, as disclosed in Published Unexamined Japanese Patent Application No. 64-35514.
This apparatus for manufacturing the metal tube armored optical fiber cable forms a continuously fed flat metal strip into a metal tube having a longitudinal gap at a top portion. A guide tube is inserted into the metal tube through this gap of the metal tube, and an optical fiber is guided into the metal tube through the guide tube. After the gap of the metal tube having received this optical fiber is closed, the metal tube is supplied to a laser welding unit.
The laser welding unit causes a guide roller to align the abutment edge portions of the top portion of the metal tube to each other. A laser beam having a focal point at a position outside the range of the abutment portions is radiated to weld the abutment portions. Since the laser beam is focused outside the rang of the abutment portions, the abutment portions can be welded without protecting the optical fiber with a heat-shielding member.
This metal tube containing the optical fiber cable is drawn to have a predetermined outer diameter, and the drawn tube is continuously wound around a capstan.
During drawing of this metal tube, an inert gas is supplied to the guide tube to carry the optical fiber cable by the viscosity resistance of the gas. While the metal tube is kept engaged with the capstan, the optical fiber cable is blown outward against the inner surface of the metal tube, so that the length of the optical fiber cable is set larger than that of the metal tube. The optical fiber cable is not kept taut to prevent the optical fiber cable from stress caused by an installation tension or the like.
In order to protect the optical fiber cable from water entering from a hole formed in a damaged metal tube, a gel is injected inside the metal tube. More specifically, after the optical fiber cable is blown outward against the inner surface of the metal tube by the inert gas at the capstan, the gel is injected from a gel guide tube different from the guide tube for guiding the optical fiber cable.
Optical fiber cables are used in a variety of application conditions and at various temperatures. The thermal expansion coefficient of the metal tube is much larger than that of the optical fiber cable. For this reason, when optical fiber cables are used at high temperatures, a tension acts on the optical fiber cable due to a difference in elongations of the metal tube and the optical fiber cable to damage the optical fiber cable. This also occurs when a cable is installed at a high tension, e.g., in installment at the bottom of a sea.
To the contrary, when optical fiber cables are used at low temperatures, the optical fiber cable is brought into contact with the inner wall surface of the metal tube having a large shrinkage amount due to a large difference between the degrees of shrinkage of the metal tube and the optical fiber cable. The optical fiber cable directly receives a side pressure from the inner wall of the metal tube. Irregular bending forces having short periods act on the optical fiber cable to cause a so-called microbend loss, thereby attenuating a signal transmitted through the optical fiber cable.
In order to prevent damage and the like, the optical fiber cable is blown outward against the inner wall surface of the metal tube while the metal tube is kept engaged with the capstan, so that the length of the optical fiber cable is set larger than that of the metal tube after the cable is straightened for use.
In this case, however, a difference between the length of the optical fiber cable and the length of the metal tube (to be referred to as an extra length hereinafter) is determined by the outer diameter of the capstan and a difference between the inner diameter of the metal tube and the outer diameter of the optical fiber cable. The extra length cannot be arbitrarily controlled, and the optical fiber cable may be damaged depending on application conditions.
As described above, while the metal tube is kept engaged with the capstan, the optical fiber cable is blown outward against the inner surface of the metal tube by an inert gas to provide an extra length to the optical fiber cable. For this reason, when a gel is to be injected into the metal pipe it must be injected while the optical fiber cable is kept blown outward against the inner wall of the metal tube due to the following reason. That is, even if the gel is injected and then the inert gas is supplied, the gel causes resistance to fail to give the extra length to the optical fiber cable. When a gel is to be injected, a gel guide tube is required in addition to the optical fiber cable and the inert gas guide tube. Since these two guide tubes must be simultaneously inserted into the metal tube, the inner diameter of the metal tube is increased. In order to obtain a thin tube from this tube, the drawing amount is increased. Metal tubes may not be occasionally thinned in accordance with diameters of optical fiber cables, resulting in inconvenience.
[Disclosure of Invention]
The present invention has been made to solve the above drawbacks, and has as its object to provide an apparatus for manufacturing a metal tube covered optical fiber cable and a method therefor, capable of preventing damage to an optical fiber cable during welding abutment portions of a metal tube, capable of continuing the manufacturing operation for a long period of time, and capable of arbitrarily obtaining an extra length.
There is provided an apparatus for manufacturing a metal tube covered optical fiber cable, comprising an assembly, having a plurality of roller pairs, for causing both side edges of a metal strip to about against each other to form the metal strip into a metal tube, laser welding means for radiating a laser beam to abutment portions of the metal tube to bond the abutment portions to obtain a sealed metal tube, and optical fiber guiding means for guiding an optical fiber or an optical fiber bundle into the formed metal tube, characterized by comprising extra length control means comprising,
tension adjusting means, arranged in the upstream of the assembly, for variably changing a tension of the metal strip in the upstream of the assembly and adjusting a tension of the metal tube,
tension adjusting means, arranged at an optical fiber guide inlet port of the optical fiber guide means, for variably adjusting a tension of the optical fiber cable, and
traction means having tension variable means for reducing a tension of the metal tube covered optical fiber cable and supplying the metal tube covered optical fiber cable.
The traction means preferably continuously draws the metal strip, the formed metal tube, and the sealed metal tube incorporating the optical fiber or optical fiber bundle through the assembly, the optical fiber guide means, the laser welding means, and drawing means, thereby reducing the tension of the metal tube covered optical fiber cable.
The tension variable means preferably comprises a capstan around which the metal tube covered optical fiber cable is wound a plurality of times.
In addition, the tension of the metal tube covered optical fiber cable at an outlet of the tension variable means is preferably adjusted by the tension adjusting means arranged in the downstream of the tension variable means.
Furthermore, the tension of the metal tube covered optical fiber cable at an inlet side of the tension variable means is preferably adjusted by tension means arranged in the upstream of the tension variable means.
Extra length control is performed such that the tension of the optical fiber cable is adjusted to a predetermined value and the tension of the metal strip is set variable, or the tension of the metal strip is adjusted to a predetermined value and the tension of the optical fiber cable is set variable, thereby obtaining an arbitrary extra length.
According to the present invention, there is provided a method of manufacturing a metal tube covered optical fiber cable, comprising the forming step of forming a metal strip subjected to traction into a metal tube through forming rollers, the laser welding step of welding abutment portions of the formed metal tube with a laser beam and forming the formed metal tube into a sealed metal tube, and the optical fiber guide step of guiding an optical fiber or optical fiber bundle in the sealed metal tube, comprising,
setting a tension of the metal strip before assembly step variable to adjust a tension of the metal tube after a drawing step, and setting a tension of the optical fiber cable in the optical fiber guide step variable and adjusting the tension of the optical fiber cable guided into the metal tube,
in the traction step, reducing tensions of the metal tube and the optical fiber cable to control an extra length of the metal tube cover optical fiber cable.
In addition, it is preferable to reduce the tension of the metal tube optical fiber cable while the metal tube optical fiber is kept subjected to traction with a capstan around which the metal tube covered optical fiber cable is wound a plurality of times.
In addition, the step of reducing the tension of the metal tube covered optical fiber may be preferably performed after a tension is applied to the metal tube.
According to the present invention, when the metal tube covered optical fiber cable is to be manufactured, the tension of the metal tube covered optical fiber at the inlet side of the tension variable means is adjusted by the tension of the metal strip formed into the metal tube and the tension of the optical fiber cable guide into the metal tube to provide a difference between the tensions of the sealed metal tube and the optical fiber cable therein. The difference in tension is reduced by the tension variable means to obtain a difference between an elongation of the sealed metal tube at the inlet side of the tension variable means and an elongation of the optical fiber cable in the metal tube. Therefore, the length of the optical fiber cable relative to the metal tube is arbitrarily adjusted by the difference in elongation.
In addition, since the capstan around which the metal tube covered optical fiber cable is wound a plurality of times is used as the tension variable means, the tension can be easily reduced while the metal tube covered optical fiber cable is kept subjected to traction.
Furthermore, the tension of the metal tube at the inlet or outlet side of the tension variable means is set variable to increase an extra length control range.